1. Field of the Invention
The invention relates to the field of protection of electrical devices such as electrical machines or transformers. It relates in particular to a method for detection of a ground fault, which occurs in the vicinity of a neutral point, in a polyphase electrical device, which is operated at a fundamental frequency. It also relates in particular to an apparatus for carrying out the method as well as to an application of the method.
2. Brief Description of the Related Art
A large number of methods are known from the prior art, by means of which ground faults or ground shorts in the windings of rotating machines such as synchronous generators are detected, and the machine can thus be protected against the negative effects of ground faults. These methods include the use of an overvoltage or overcurrent relay at the neutral point, a zero voltage (zero sequence) overvoltage relay, or a residual current differential protection circuit.
Ground fault protection for a generator based on overvoltage or overcurrent determination results in simple and reliable protection. However, it has one major disadvantage: ground faults which occur in the windings close to the neutral point of the generator are not detected. It has therefore already been proposed (Charles J. Mozina, Upgrading Hydroelectric Generator Protection Using Digital Technology, Waterpower '97, Atlanta Ga., Aug. 5–8 (1997)) for one hundred percent ground fault protection to be achieved by combining a conventional overvoltage protection circuit (for 95% of the stator winding) with an undervoltage protection circuit which is tuned to the third harmonic of the generator fundamental frequency (for the remaining 5% of the winding close to the neutral point). This proposal is based on the fact that, in many synchronous generators, the inducted electromotive voltage at the neutral point contains higher harmonics, which produce a corresponding high harmonic current in the connection from the neutral point to ground, in general a resistance. If an earth fault occurs in the vicinity of the neutral point, it acts as a bypass for this resistance. The reduction in the voltage drop which results from the bypass can then be detected as an undervoltage. These known ground fault protection methods are supported by the use of step-up transformers between the generator and the power supply system, which allow the generator to be isolated from the power supply system with regard to grounding.
However, recently, power station configurations are being increasingly used in which there is no need for the conventional step-up transformers, since the generators produce the power supply system voltage directly. This is achieved by means of a special winding technology, in which the windings of the generator use high-voltage cables. Generators such as these have become known by the name “Powerformer” (see, for example, M. Leijon, “Powerformer—a radically new rotating machine”, ABB review 2/98, pp. 21–26 (1998)).
It has now been found that the known methods for detection of ground faults in these rotating machines and transformers which are designed using the new winding technique do not operate with one hundred percent reliability.